The humor is simply the unexpected result that only the bag was hitchhiking, and not Horace. This explains why he is still smiling in the third panel. I also thought that part of the joke was that he had to wear a glove not only in order to show a thumb to signal the car to stop, but also to wave farewell.
P.S. If Horace had been angry at not getting a ride, he could have altered the glove to extend a different finger.
Comics I Don't Understand 
It is the bag that is going to the next city, not Horace.
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I had to read it three times before I figured that out.
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Ok, but what’s the joke?
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The humor is simply the unexpected result that only the bag was hitchhiking, and not Horace. This explains why he is still smiling in the third panel. I also thought that part of the joke was that he had to wear a glove not only in order to show a thumb to signal the car to stop, but also to wave farewell.
P.S. If Horace had been angry at not getting a ride, he could have altered the glove to extend a different finger.
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Kilby, I didn’t think about Horace needing a glove because he regularly plays intricate guitar runs with his hooves.
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Oh, and to Padraig’s point, I just realized why he needed the glove. I see that as a secondary joke.
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“Joke?”
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Perhaps “oddity” would be more fitting.
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This is one way to “get rid of your baggage.”
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